This invention relates to viewing and imaging systems for particular, though not exclusive, application in the field of screen printers for applying solder paste to printed circuit boards. The following description will focus on screen printer applications, but it will be understood that the various aspects of the invention find application in other areas where similar techniques are used.
In screen printers for printed circuit boards, the screen is positioned over the circuit board and solder paste is applied to the board through apertures in the screen. To ensure that the solder paste is printed at the correct locations on the board for subsequent component placement, the screen must be aligned with the board prior to printingxe2x80x9d To perform the alignment, an imaging device such as a video probe is generally moved between the screen and board to view reference marks, or xe2x80x9cfiducialsxe2x80x9d, at corresponding positions on the screen and board. Images of corresponding fiducials on the screen and board are relayed to vision processing apparatus. The vision processing apparatus determines the relative mis-alignment of the screen and board from the positions of the fiducials in the acquired images, and the relative position of the screen and board is adjusted to achieve alignment to prior to printing.
The general arrangement of one type of screen printer is illustrated in FIG. 1 of the accompanying drawings. The circuit board 1 to be printed is supported by a base (not shown) of the printer in a generally horizontal plane parallel to the x and y axes shown in the figure. A screen 2 is supported in a frame 2a so as to lie above, and generally parallel to, the board 1. The position of the screen 2 in the xy plane can be adjusted by means of screen positioning motors 3 the operation of which is controlled by a position controller 4. The support frame 2a and screen positioning motors 3 are mounted in a printhead portion (not shown) of the printer which can be pivoted away from the base to allow access when necessary, for example to adjust the board support. The apparatus includes an imaging device 5 which is mounted on an XY table (not shown) for movement in a horizontal plane between the board 1 and screen 2. The position of the imaging device 5 is also controlled by the position controller 4.
The surface of the screen 2 facing the board 1 has a plurality of reference marks, or screen fiducials, 6a and 6b thereon. The surface of the board 1 facing the screen 2 has corresponding board fiducials 7a and 7b. The positions of the board and screen fiducials are such that when each pair of corresponding fiducials 6a, 7a and 6b, 7b are in alignment, the screen is correctly aligned relative to the board for the subsequent printing operation to be performed after simply raising the board into contact with the screen by means of a mechanism provided in the base of the printer.
The imaging device 5 is operated so as to acquire images of the fiducials 6, 7 and the acquired images are supplied to a vision processor 8 which is programmed to determine the locations of the fiducials in the images. The fiducial location data is then supplied to the position controller 4 for adjusting the position of the screen 2 to bring the corresponding fiducials 6a, 7a and 6b, 7b, and hence the board and screen, into alignment. A video monitor 9a allows an operator to monitor images supplied to the vision processor 8. An operator interface 9b is connected to the position controller 4 to allow operator control of the apparatus, eg. during preliminary set up procedures.
The general arrangement of a known imaging device 5 is illustrated schematically in FIG. 2 of the drawings. Here, the imaging device is a video probe comprising a CCD camera 11 and an optical system indicated generally at 12. The optical system 12 comprises screen and board lighting indicated schematically at 13 and 14 respectively. The screen and board lighting 13 and 14 constitutes xe2x80x9cdirect lightingxe2x80x9d for illuminating the regions A and B of the screen and board with substantially collimated light as indicated by the arrows in the figure. The direct lighting 13, 14 can be implemented in a number of ways, for example by optical arrangements which receive light from sources at one side thereof and reflect the light upwardly/downwardly towards the screen/board. As indicated by the bold arrows in the figure, light reflected from the screen and board is incident on reflecting means 16 in a return light channel 17 of the system which is indicated schematically by the broken lines in the figure. The reflecting means 16 may be, for example, a prism arrangement which reflects incident light from the screen/board along the return light channel 17. The return light is transmitted along the channel 17 via various optics (not shown) to the CCD camera 11. The illuminated regions of the screen and board may be imaged side by side on the CCD array.
While the direct light, which is nominally Perpendicular to the screen and board, is sufficient for viewing fiducials for alignment purposes, if the probe 5 is also to be used for inspection purposes then it is desirable for additional, diffuse lighting to be provided. In particular, if the probe is to be used to inspect the screen 2 for aperture blockage or contamination, or to inspect the board 1 for missing, misaligned or excess print, then the direct light may be insufficient to enable the required features to be distinguished. However, the location of the direct screen and board lighting 13, 14 above and below the return light channel 17 as shown in FIG. 2 means that in practice there is inadequate space to provide the required diffuse lighting on the probe if a practical compact design is to be achieved.
According to one aspect of the present invention there is provided viewing apparatus for location between a screen and board in a screen printer for viewing opposing regions of the screen and board, the apparatus comprising:
reflecting means for reflecting incident light from the screen and board along a return light channel;
beamsplitter means in the return light channel for transmission of incident light from the reflecting means along the return light channel:
lighting means, comprising first and second light sources, for illuminating the beamsplitter means, the beamsplitter means being arranged for reflecting light from the first and second light sources towards the reflecting means and the reflecting means being arranged for further reflecting the light from the first and second light sources towards the screen and board respectively, and
a light barrier, extending between the beamsplitter and reflecting means, for inhibiting cross-talk between light from the first and second light sources reflected by the beamsplitter means.
In embodiments of this aspect of the invention, therefore, the light for illuminating the screen and board is introduced into the return light channel while cross-talk between the screen and board lighting is inhibited by the light barrier. This frees the areas occupied by the screen and board lighting 11, 14 in the arrangement of FIG. 2, so that a compact design can be achieved and the regions occupied by the screen and board lighting 13, 14 in FIG. 2 can be used for diffuse lighting means if desired.
While arrangements can be envisaged where the light from the first and second light sources is introduced into the return light channel from above (screen side) and below (board side), it is preferred that the lighting means is located laterally of the return light channel. Further, while arrangements can be envisaged where the light from the first and second light sources is introduced into the return light channel from opposite sides thereof, it is preferred that the lighting means is located on one side only of the return light channel so that light from both light sources is introduced from the same side. With this arrangement at least, it is preferred that the light barrier further extends between the lighting means and the beamsplitter means to inhibit cross-talk between light from the first and second light sources incident on the beamsplitter means.
Depending on the particular arrangement of the optical system, the light barrier may comprise a thin membrane, such as a thin metal sheet or a layer or coating of some other suitable material. The membrane is preferably substantially non-reflective. In preferred embodiments, the light barrier is generally perpendicular to the direction of light reflected towards the screen and board by the reflecting means, and is most preferably substantially aligned with the optical axis of the return light channel.
Through provision of the light barrier, in operation of the lighting means the screen is illuminated substantially only by the first light source, and the board is illuminated substantially only by the second light source. This facilitates efficient operation and is particularly important where, as in preferred embodiments, the light sources are independently operable and/or adjustable. For example, if during set-up the operator adjusts the intensity of one light source to optimise, say, the screen lighting, the adjustment will not effect the board lighting and vice versa.
Referring again to FIG. 2, in some cases the screen and board lighting 13, 14 may be strobed to instantaneously illuminate the screen and board fields of view. This allows fiducial capture (ie. acquisition of fiducial images to be performed while the probe is in motion between the screen and board. Of course, while the provision of suitable optics in the return channel 17 of FIG. 2 allows the screen and board to be viewed simultaneously, in other arrangements where a probe looks only in one direction, or only one direction at a time, it may also be desirable to strobe or pulse the lighting. While strobe lighting might be used in normal operation for fiducial capture, in practice continuous lighting is also required, eg. during set-up to allow the operator to view the video image on the monitor 9a (FIG. 1) and perform the necessary set-up procedures. However, continuous lighting gives a different lighting effect to the strobe lighting used during fiducial capture, and this can make it difficult to optimise the various system parameters during set-up. A further drawback of continuous lighting generally is that it can result in image blur when the probe is in motion, making, precision measurements difficult or impossible.
According to another aspect of the present invention there is provided a video probe for generating a video signal representing images of regions of a screen and/or board in a screen printer, the video probe comprising:
lighting means for illuminating regions to be viewed;
control means for controlling the lighting means to generate pulses of light; and
imaging means for acquiring images of the regions illuminated by the light pulses and generating a video signal representing the images so acquired,
wherein the control means is operable in a first mode to control the lighting means to generate light pulses repeatedly at a sufficient rate to produce a video signal which, when displayed to an operator, is perceived as a substantially continuous picture.
In accordance with this aspect of the Invention, therefore, a pseudo-continuous lighting effect is achieved by using repetitive light pulses. An operator monitoring the video signal therefore sees a substantially continuous video picture similar to prior arrangements where true continuous lighting was used. However, since the lighting is in fact pulsed, the effect of probe movement on the resulting images can be substantially reduced as compared with true continuous lighting. Further, where the lighting means will be strobed to capture a single image during normal operation, eg. for fiducial capture, the lighting effect of the repetitive pulses will be much closer to the conditions during normal operation.
To avoid flicker in the displayed image, the repetitive light pulses are preferably generated in synchronism with the video signal. It is particularly preferred that the pulses are generated at the frame rate of the video signal for conformity with the conditions during normal operation, where one frame is acquired from one pulse of light, and to provide a high quality video picture for monitoring, though a lower pulse rate might produce an apparently continuous video image which is acceptable for some purposes.
As indicated, the lighting means may be the direct lighting used for fiducial capture in normal operation. Alternatively or in addition, the lighting means may be diffuse lighting means used for inspection purposes, allowing inspection to be performed with the probe in motion. In automated inspection processes, the diffuse lighting may also be triggered for single image capture. In both cases, therefore, the control means may be operable in a second mode to control the lighting means to generate pulses of light at times dependent upon external control signals received by the control means. This second, triggered mode can then be used for single image capture when required.
Where a plurality of light sources are used in the imaging device, it may be desirable for these to be of different intensity, and further to be adjustable in intensity. For example, independent light sources may be used for direct lighting of the screen and board. As a further example, if the imaging device is to be used for inspection as well as for fiducial capture, as discussed above it is desirable to provide additional diffuse lighting. This may be of different intensity to the direct lighting used for fiducial capture, and may itself comprise a plurality of separate light sources to provide light from varying angles.
According to a further aspect of the present invention there is provided imaging apparatus for acquiring images of regions of a screen and/or board in a screen printer, the apparatus comprising:
a plurality of lighting means for illuminating regions to be imaged;
control means for controlling each lighting means to generate a pulse of light: and
imaging means for acquiring images of the regions illuminated on activation of the lighting means,
wherein the control means is operable to control the plurality of lighting means to generate respective pulses of different durations such that the temporal midpoints of pulses from respective lighting means are coincident.
In embodiments of this aspect of the invention, therefore, the imaging device can include a plurality of lighting means which can generate pulses of different durations, ie. different temporal pulse widths, but though the pulse widths for different lighting means are different, the pulses are centred at the same point in time. Since the effective intensity of light from different lighting means is dependent on the pulse width, this allows the various lighting means to produce different intensity light in a strobe situation, the coincident light pulses xe2x80x9cfreezingxe2x80x9d the image at a single point in time.
The plurality of lighting means may comprise direct lighting means and/or diffuse lighting means such as may be used for inspection purposes. Further in preferred embodiments the control means is operable to vary the pulse duration, and hence effective intensity, of one or more, and preferably all, of the lighting means in response to a control input. Thus, the intensity of individual lighting means can be varied independently of the others, while the light pulses from different sources are still centred on the same point in time by the control means.
As in embodiments of the second aspect of the invention, the control means may be operable in a first mode to control the lighting means to generate light pulses repeatedly, preferably in synchronism with a video signal output by the imaging means, and most preferably at the frame rate of the video signal. The control means may also be operable in second mode to trigger the lighting means to generate pulses at times dependent on external control signals received the control means.
In general, systems may embody more than one of the various aspects of the invention discussed above. Moreover, when features are described herein with reference to apparatus according to the invention, corresponding features may be provided in a method of the invention, and vice versa.